1. Field of the Invention
The present invention relates to a light-emitting element having an optical resonance structure, an image forming apparatus, an image display apparatus and an image reading apparatus comprising the light-emitting element.
2. Description of Related Art
As a conventional image forming apparatus comprising a light-emitting element of this kind, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2000-77184 is known. In the image forming apparatus, toner images in different colors are formed on corresponding photoreceptor drums by a conventional electrophotographic process.
In an electrophotographic process, a photoreceptor drum is exposed to light emitted from an exposure device, and an electrostatic latent image is formed on the photoreceptor drum. The exposure device comprises a light-emitting section 58 having an optical resonance structure as shown by FIG. 21 and a control circuit 9. The light-emitting section 58 generally comprises a first transparent portion 81 made of glass or the like, a cathode 82 made of aluminum or the like, an organic compound 83, an anode (ITO=indium tin oxide) 84, a plurality of translucent reflecting portions 85 (shown as 85A through 85C), and a second transparent portion 86 made of glass or the like. On the transparent section 81 serving as a substrate, the cathode 82, the organic compound 83, the anode 84 and the translucent reflecting portions 85A through 85C are stacked in this order, and on the top thereof, the transparent portion 86 is provided.
The control section 9 applies electric voltages to the cathode 82 and the anode 84. In response, electrons are injected in the organic compound 83 from the cathode 82, and holes are injected in the organic compound 83 from the anode 84. In the organic compound 83 between the layers 82 and 84, the holes and the electrons couple with each other, and light is emitted. The translucent reflecting sections 85 and the cathode 82 form a microstructure functioning as an optical resonator. Reflection and reciprocation of photons between these layers 82 and 85 induce resonance, and high-intensity light is emitted from the translucent reflecting portions 85 in a specified direction (toward the photoreceptor drum).
The upper section of FIG. 22 shows light distribution curves of the light-emitting section 58, and the lower section of FIG. 22 shows light distribution curves of a light-emitting device not having an optical resonance structure (which will be hereinafter referred to as a comparative example). Each of the light distribution curves is drawn with respect to each wavelength range. Each of the light distribution curves for each wavelength range shows the intensities of light radiating from a light-emitting point O in various directions. In FIG. 22, a line N showing the optical axis of the light-emitting section 58 is drawn virtually as a reference of directions. The directions are shown as degrees at which the reference line N is rotated on the light-emitting point O. The intensities of light radiating in various directions are shown as distances between the light-emitting point O and the respective points on the corresponding curve.
Both the light-emitting section 58 shown by FIG. 21 and the comparative example are operable to emit light within a wavelength range from 620 nm to 650 nm. In the upper section of FIG. 22, the curve denoted by f1 is a light distribution curve showing the distribution of light within this wavelength range emitted from the light-emitting section 58. In the lower section of FIG. 22, the curve denoted by f2 is a light distribution curve of light within this wavelength range emitted from the comparative example. In the upper section and in the lower section of FIG. 22, the same polar coordinate system is used. As is apparent from the light distribution curves f1 and f2, the light-emitting section 58 emits higher-intensity light in the direction of 0 degrees (in the direction of optical axis) than the comparative example. The same tendency is seen with respect to a wavelength range from 650 nm to 680 nm (see the light distribution curves g1 and g2) and with respect to wavelength range from 680 nm to 710 nm (see the light distribution curves h1 and h2).
However, such a light-emitting element having an optical resonance structure changes its light distribution characteristic and spectral radiance characteristic with changes in ambient temperature.
Accordingly, an image forming apparatus employing such a light-emitting element has a problem that the image density varies with changes in ambient temperature due to the temperature characteristic of the light-emitting element of emitting light with spectral radiance changeable with changes in temperature. In the following, this problem is described with reference to FIGS. 23 through 25.
The left section of FIG. 23 shows distribution of light emitted from such a light-emitting element and spectral radiance of the light traveling to a photoreceptor drum when the ambient temperature is low. The right section of FIG. 23 shows distribution of light emitted from the light-emitting element and spectral radiance of the light traveling to the photoreceptor drum when the ambient temperature is high. As is apparent from FIG. 23, the radiance around the wavelength of 630 nm is great under low temperature, while the radiance around the wavelength of 580 nm is great under high temperature. The photoreceptor drum has a spectral sensitivity characteristic as shown by FIG. 24, and the sensitivity of the photoreceptor drum varies depending on the wavelength of light radiated thereto. Therefore, as shown by FIG. 25, with a change in ambient temperature around the light-emitting element, the surface potential of the photoreceptor drum after exposure changes, and consequently, the image density changes.
Such a light-emitting element having an optical resonance structure may be used not only in an image forming apparatus but also in an image display apparatus as a pixel and in an image reading apparatus as an element of an illuminating device operable to illuminate a document. When the light-emitting element is used for these purposes, changes in ambient temperature affect the display colors of the image display apparatus or the image data produced by the image reading apparatus.